Landing Cost

I have a scenario regarding the unplanned delivery cost. I have to raise a PO to the main vendor with the base price and the discount. But one landing cost is also there which we do not know at the time of making PO.This landing cost we have to pay to another vendor who is forwarding the goods to our plant.And the remaining amount of the PO to the main vendor. I have a idea how to do this,but I want to know the step by step Procedure for this.

Do we need to create the partner functions, if yes then please explain?